The applicant of this application has proposed a capacitive physical-quantity detection apparatus for detecting a physical quantity on the basis of the capacitance between a pair of electrodes disposed so as to face each other as disclosed in JP-A-2000-81449.
According to this capacitive physical-quantity detection apparatus, a capacitive variation between a movable electrode displaced in accordance with variation of a physical quantity (for example, acceleration) and a fixed electrode disposed so as to face the movable electrode is picked up as a voltage variation in a C-V converting circuit designed as a switched capacitor and then subjected to signal processing, whereby the signal corresponding to the variation of the physical quantity can be output.
Furthermore, a self-diagnosing signal (predetermined voltage) is periodically applied between the movable electrode and the fixed electrode to induce electrostatic force between the movable electrode and the fixed electrode, thereby setting the capacitive physical-quantity detection apparatus to such a state that a pseudo physical quantity is applied to the movable electrode. That is, self-diagnosis can be carried out.
However, in the case of a construction that moisture is detected as a physical quantity, a pair of electrodes disposed so as to face each other are fixedly disposed, and thus even when a predetermined voltage is forcedly applied between the pair of electrodes, it is difficult to set the apparatus to such a state that a pseudo physical quantity is induced. That is, the self-diagnosis cannot be carried out.